The present invention relates to mechanical seals, and more particularly to a mechanical seal which is adapted to seal a pump or other rotary shaft against leakage of high pressure fluids in the pump housing.
In investigating high pressure mechanical face seals, it was discovered that the originally flat seal surfaces will become distorted in use under the simultaneous effects of hydrostatic load and temperature gradient; this latter condition being caused by the dissipation of the friction heat at the sealing faces. Once a seal is installed, very little can be done about its performance; which is reflected by the developing friction torque, leakage and wear rate of surfaces for any given load-speed condition. A sealing member in the form of a cylindrical body with a constant wall thickness and having a constant hydrostatic load on the full exterior length of the sealing member results in the ring faces at the ends of the seal remaining planar. However, a partial hydrostatic load exerted adjacent one end of the sealing member will result in a concave face at the end of the body adjacent the hydrostatic load and a convex face at the opposite end. Also, where there is a thermal load in an axial direction of the cylindrical body with one end of the member at a higher temperature than the opposite end, the end of the sealing body adjacent the higher temperature will have a convex sealing surface while the face at the lower temperature end will have a concave sealing surface.
Every mechanical seal requires at least two sealing rings with two adjacent surfaces in sealing contact. Distortion may occur on either or both seal rings which would result in one of several basic relationships. Where there is a constant hydrostatic load along the external surfaces of both sealing rings, the adjacent sealing surfaces between the two rings will remain in a full face contact. Where one sealing ring remains cylindrical while the other sealing ring is distorted, either by a partial hydraulic load or an axial thermal load, resulting in a concave sealing face, the actual sealing contact will occur along the outer diameter area of the sealing faces. On the other hand, if one sealing ring is distorted to present a convex sealing face adjacent the undistorted ring, the sealing faces will be engaged along an internal diameter contacting area. If both sealing rings are distorted so that one sealing ring provides a concave sealing face and the other sealing ring provides a convex sealing face adjacent the concave face, a full face sealing contact will result.
Considering the fact that high pressure seal designs are normally associated with some degree of hydrostatic balancing, usually in the range of 60 to 90 percent, it is clear that in the case of a full face contact, the face contact pressure will be equal to the balance pressure. For an outer diameter face contact, the face contact pressure may be considerably larger than the balance pressure; while an inner diameter face contact will yield a normally smaller face contact pressure than either of the other two arrangements.
For any given seal system, an important goal is to reduce the friction heat occuring from the sliding contact of the sealing faces, and the obvious way to achieve this goal is to reduce the face contact pressure to minimum safe level. An important result of this will be a reduction in the heat distortion of the sealing rings. The present invention provides a mechanical seal design which will assure adjustment of the deformation of the seal faces after the seal has been installed and is functioning in the requisite structure. Such a sealing arrangement can be used for a rotating shaft of a pump or similar device whereby the pressure occurring at the intake and delivery sides of the pump are utilized to provide the externally controlled deformation of the seal face for the stationary sealing ring. This arrangement includes the establishment of hydrostatic pressure upon one or more areas of the exterior surface of the stationary seal ring so as to provide a desired deformation of the sealing ring to overcome deformation caused by either hydraulic or thermal conditions within the ring and at the rotating shaft.
The present invention comprehends a sealing ring arrangement wherein it is possible to provide conditions at will that would result in an inner diameter face contact, a full face contact, or an outer diameter face contact of the sealing faces. Also, a full face contact condition can now be changed to an inner diameter contact or an outer diameter contact depending on the conditions within the system. This system provides a rotary sealing ring mounted in a seal ring holder surrounding the rotating shaft, a stationary sealing ring mounted within a gland plate, and appropriate piping and valves to provide either a low pressure, a high pressure or an atmospheric pressure condition to the stationary sealing ring. The low and high pressure conditions can be appropriately obtained from a pump that is operated by the rotating shaft. The stationary sealing ring is preferably formed of a material having a low thermal expansion coefficient, while the rotary sealing ring may be of a hard metal or other suitable material which has a higher thermal expansion coefficient.
The present invention further comprehends a stationary sealing ring arrangement for use with a rotary sealing ring to provide appropriate face sealing contact wherein the cylindrical outer surface of the stationary sealing ring is divided into a pair of axially spaced areas separated by sealing O-rings so as to be hermetically sealed off from direct communication with the seal cavity containing a high pressure fluid. Each of the two areas, however, is connected through the proper piping and valves either to atmospheric pressure or to the intake side of the pump operated by the rotating shaft or to the delivery sides of the pump; thus providing three conditions of hydrostatic pressure. By appropriate control of the valves to the two spaced areas, an infinite number of co-working seal face configurations can be produced due to the possible variation of the hydrostatic loads applied to either area between atmospheric and up to delivery pressure. Thus, this arrangement is well suited to counterbalance thermal and hydrostatic load deformation of the rotating ring which, in an extreme case, may reach unsafe proportions, and separate the sealing faces. By controlling the sealing contact between the rotary and stationary rings, the friction heat caused by the contact of the rotating sealing surfaces will be reduced to decrease heat distortion thereof, and excessive wear of the sealing faces is also reduced to a minimum.
Further objects are to provide a construction of maximum simplicity, efficiency, economy, and ease of assembly and operation, and such further objects, advantages and capabilities as will later more fully appear and are inherently possessed thereby.